Radio frequency digital to analog converter

ABSTRACT

A RF digital to analog converter has a first capacitor arrangement, a first common node, and a first controller. The first capacitor arrangement has multiple switchable capacitor paths arranged in parallel. Respective switchable capacitor paths have a switchable element and a capacitor coupled in series. The first common node is connected to the multiple switchable capacitor paths. The first controller receives a baseband signal having an in-phase component and a quadrature component, and a local oscillator (LO) signal having an in-phase LO signal and a quadrature LO signal. The first controller combines the in-phase component and the in-phase LO signal to obtain a first in-phase modulation signal and combines the quadrature component and the quadrature LO signal to obtain a first quadrature modulation signal. The first controller controls the multiple switchable capacitor paths of the first capacitor arrangement with the first in-phase modulation signal and/or the first quadrature modulation signal.

FIELD

Implementations of the present invention relate to a radio frequencydigital to analog converter and to a method for converting a digitalsignal to an analog signal.

BACKGROUND

Digital to analog converters (DACs=Digital to Analog Converters) may beused as modulators (e.g. as polar modulators for AM signals or as vectormodulators for IQ signals). Modulators applied to transmitters of mobilecommunication devices serve the purpose to transfer a digital basebandsignal (e.g. a data signal or audio signal) via a carrier to the basestation of the communication network. In detail, the baseband signal ismodulated onto the carrier, which may, for example, have a frequency of1.8 or 2.1 MHz (depending on the network standard, cf. LTE or UMTS),such that the radio frequency output by the modulator of the transmittercomplies with the frequency of the carrier and comprises the content ofthe baseband signal.

The group of currently used high-frequency DAC (mixers) can be brokendown into a first subgroup of modulators configured to modulate a polarsignal to a carrier and into a second subgroup of modulators configuredto modulate an IQ signal to the carrier. Furthermore, the singularmodulator types differ regarding its bandwidth, its efficiency (currentconsumption) and regarding additional features like MIMO capability(Multiple Input, Multiple Output). Currently, none of the knownmodulator types conjuncts a high signal bandwidth, a low currentconsumption and MIMO capability. Therefore, there is a need for animproved approach.

SUMMARY

A radio frequency digital to analog converter comprises a first capacityarrangement, a first common node and a first controller. The firstcapacitor arrangement comprises a plurality of switchable capacitorpaths arranged in parallel, wherein each switchable capacitor pathcomprises a switchable element and a capacitor coupled in series. Theplurality of switchable capacitor paths of the first capacitorarrangement is connected to the first common node. The first controlleris configured to receive a baseband signal having an in-phase componentand a quadrature component, and to receive an LO (local oscillator)signal having an in-phase LO signal and a quadrature LO signal. Thefirst controller is further configured to combine the in-phase componentand the in-phase LO signal to obtain a first in-phase modulation signal,and to combine the quadrature component and the quadrature LO signal toobtain a first quadrature modulation signal. Furthermore, the firstcontroller is configured to control a plurality of switchable capacitorpaths of the first capacitor arrangement in accordance with the firstin-phase modulation signal and/or in accordance with the firstquadrature modulation signal.

A radio frequency digital to analog converter comprises a firstcapacitor arrangement comprising a plurality of switchable capacitorpaths and a second capacitor arrangement comprising a further pluralityof switchable capacitor paths, each switchable capacitor path comprisesan inverter and a capacitor coupled in series, wherein the switchablecapacitor paths of the first capacitor arrangement are arranged inparallel and coupled to a first common node via the respectivecapacitors and wherein the switchable capacitor paths of the secondcapacitor arrangement are arranged in parallel and coupled to a secondcommon node via the respective capacitors. The radio frequency digitalto analog converter further comprises an RF-balun arranged between thefirst common node and the second common node, as well as a first and asecond controller. The first and the second controller are configured toreceive a baseband signal having an in-phase component and a quadraturecomponent and to receive an LO signal having an in-phase LO signal and aquadrature LO signal. The first LO is configured to combine the in-phasecomponent and the in-phase LO signal to obtain the first in-phasemodulation signal, and to combine the quadrature component and thequadrature LO signal to obtain a first quadrature modulation signal,wherein the first controller is further configured to control theplurality of inverters of the first capacitor arrangement in accordancewith the first in-phase modulation signal and/or in accordance with thefirst quadrature modulation signals. The same controller is configuredto combine the in-phase component and the in-phase LO signal to obtain asecond in-phase modulation signal, and to combine the quadraturecomponent and the quadrature LO signal to obtain a second quadraturemodulation signal, wherein the second controller is further configuredto control the plurality of inverters of the second capacitorarrangement in accordance with the second in-phase modulation signaland/or in accordance with the second quadrature modulation signal. Theradio frequency digital to analog converter is configured to output adifferential analog signal comprising the in-phase component and thequadrature component between the first and the second common node inaccordance with the baseband signal.

A method for converting a digital signal to an analog signal comprisesthe steps of receiving a baseband signal having an in-phase componentand a quadrature component, and of receiving an LO signal having anin-phase LO signal and a quadrature LO signal. The method furthercomprises the steps of combining the in-phase component and the in-phaseLO signal to obtain a first in-phase modulation signal and of combiningthe quadrature component and the quadrature LO signal to obtain a firstquadrature modulation signal. Furthermore, the method comprises the stepof controlling a plurality of switchable capacitor paths of a firstcapacitor arrangement in accordance with the first in-phase modulationsignal and/or in accordance with the first quadrature modulation signal,wherein the first capacitor arrangement comprises the plurality ofswitchable capacitor paths arranged in parallel, wherein each switchablecapacitor path comprises a switchable element and a capacitor coupled inseries, and wherein the plurality of switchable capacitor paths of thefirst capacitor arrangement is connected via a first common node.

Further implementations refer to a mobile communication devicecomprising an above discussed radio frequency digital to analogconverter and to a transmitter of a mobile communication devicecomprising the radio frequency digital to analog converter.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the present invention will be discussed belowreferring to the enclosed drawings, wherein:

FIG. 1 shows a schematic representation of a mobile communication devicecomprising a radio frequency digital to analog converter implementedinto a transmitter of the mobile communication device;

FIG. 2 shows a block diagram of a first implementation of the radiofrequency digital to analog converter;

FIG. 3A shows a schematic block diagram of a further implementation ofthe radio frequency digital to analog converter designed for outputtinga single-ended signal;

FIG. 3B shows a schematic block diagram of a controller of the radiofrequency digital to analog converter shown in FIG. 3A;

FIG. 4A shows a schematic block diagram of an implementation of theradio frequency digital to analog converter designed for outputting adifferential signal; and

FIG. 4B shows a schematic block diagram of a controller of the radiofrequency digital to analog converter shown in FIG. 4A.

DETAILED DESCRIPTION

Different implementations of the teachings disclosed herein willsubsequently be discussed referring to FIGS. 1-4B, below. Identicalreference numbers are provided to objects having identical or similarfunctions so that objects referred to by identical reference numberswithin the different implementations are interchangeable and thedescription thereof is mutually applicable.

FIG. 1 shows a mobile communication device 10, e.g. a mobile phone,smartphone or tablet PC, comprising a transmitter 12 coupled between abaseband processor 14 and an antenna 16. The transmitter 12 comprises amodulator comprising a DAC 18 and an optional oscillator 20 (e.g. amodulated DCO (Digital Controlled Oscillator)). The DAC 18 (modulator)may be formed by a radio frequency digital to analog converter (RF-DAC).

The transmitter 12 is configured to transmit a communication signal 24(e.g. an analog radio frequency signal) over the air via the antenna 16,wherein the communication signal 24 is equal to or based on a radiofrequency signal 22 output by the DAC 18. The transmitter may optionallycomprise further signal processing units (not shown), like amplificationstages, arranged between the DAC 18 and the antenna 16 for processingthe RF signal 22. The analog RF signal 22 to be transmitted may comprisean audio signal or a data signal based on a signal 26 provided by thebaseband processor 14. The baseband processor 14 provides the basebandsignal 26 in digital form, for example as an IQ signal. The baseband IQsignal 26 comprises an in-phase component and a quadrature componentwhich should be modulated onto a carrier (e.g. a 2.1 MHz carrier) havinga frequency and bandwidth defined by the used communication networkstandard. The modulation is performed by the DAC 18 (modulator) whichcombines the baseband signal 26 with an LO signal 28 (oscillator signal)provided by the oscillator 20. It should be noted that the LO signal 28(e.g. having a frequency of 2 GHz) may be a derivate signal of a signal(e.g. having a frequency of 8 GHz) directly output by the oscillator 20.In detail, the IQ modulation, also referred to as vector modulation, isperformed by combining the first in-phase component of the basebandsignal 26 with the respective in-phase LO signal of the LO signal 28 andby combining the quadrature component of the baseband signal 26 with therespective quadrature LO signal of the LO signal 28.

Currently, multiple architectures for the DACs 18 (modulators) are useddepending on the respective requirements. For example, a directmodulator (vector modulator architecture) is mostly chosen when a largemodulation bandwidth is required (e.g. in case of carrier aggregation inLTE or WiFi 11n). However, the direct modulator architecture does notoffer the best efficiency. Another drawback is the noise behavior andthe chip area consumption of such direct modulators. An improvementregarding the efficiency may be achieved by choosing a current steeringIQ RF DAC as modulator which typically comprises a plurality of mixercells, wherein a first subplurality of these mixer cells is designed asin-phase mixer cells and a further subplurality of the mixer cells isdesigned as quadrature mixer cells. However, this current steering IQ RFDAC has a medium efficiency. Therefore, there is a need for an improvedapproach such that the efficiency of the DAC 18 (modulator) may beincreased while maintaining the high bandwidth.

Below, implementations of RF DACs (used as modulator for thecommunication device 10) enabling high efficiency will be discussed.

FIG. 2 shows an implementation of a DAC 18′ having an improvedarchitecture. The DAC 18′ comprises a first capacitor arrangement 30comprising a plurality of switchable capacitor paths 32 a and 32 b.Furthermore, the DAC 18′ comprises a first controller 34 which iscoupled to the singular switchable capacitor paths 32 a and 32 b in asignal technical manner. Therefore, the controller 34 comprises anoutput 36 a and 36 b for each switchable capacitor path 32 a and 32 b,respectively.

Respective switchable capacitor paths, 32 a and 32 b, comprise acapacitor, 38 a and 38 b, as well as a switchable element, 40 a and 40b. In each switchable capacitor path 32 a or 32 b a respective capacitor38 a or 38 b and a respective switchable element 40 a or 40 b is coupledin series. Via the switchable element 40 a and 40 b, respectively, oneside of the capacitor 38 a or 38 b may be coupled to the knownelectrical potential when enabling the switchable element 40 a or 40 b.At the second side, the plurality of capacitors 38 a and 38 b of theswitchable capacitor paths 32 a or 32 b may be connected to each othervia a common node 42 so that the plurality of switchable capacitor paths32 a and 32 b are coupled in parallel. From another point of view thatmeans that an electrical signal may be output via the common node 42wherein the amplitude of the signal depends on the number of enabledswitchable elements 40 a and 40 b. The switchable element 40 a and 40 bof each switchable capacitor path 32 a and 32 b may be formed by aninverter or by another circuit comprising one or more transistors whichenables the switchable electrical coupling of the capacitors 38 a or 38b to the known electrical potential. The switchable elements 40 a and 40b are controlled via a (binary) control signal output by the controller34 via the control outputs 36 a and 36 b.

The controller 34 controls the plurality of switchable capacitor paths,32 a and 32 b, such that an IQ modulated radio frequency signal isoutput by the capacitor arrangement 30 via the common node 42. For thispurpose, the controller 42 is configured to combine a local oscillatorsignal 28 (LO signal) and a baseband signal 26, which respectivelycomprise an in-phase component (I) and a quadrature component (Q). Thedigital baseband signal 26 is provided by the baseband processor (notshown) in a manner comparable to the so-called IF signals in case for AMmodulations. These baseband signals 26 and LO signals 28 are combined inthe digital domain, so that the LO signals 28 (originally used for phasemodulation) are no longer modulated, but provide a 0°- and/or90°-constant phase at the RF frequency. Based on the combinations of thebaseband signal 26 and the LO signal 28 a so-called in-phase modulationsignal and/or a so-called quadrature modulation signal is generated. Inaccordance with the in-phase modulation signal and/or in accordance withthe quadrature modulation signal and/or in accordance with a signalcombining these two signals the singular switchable elements 40 a and 40b are selectively enabled by outputting the respective control signalsvia the control outputs 36 a and 36 b. In other words, that means thatthe mixing function is now fully digitally implemented into thecontroller 34. Thus, the mixing performed by the controller 34 (withoutclassical mixer stage) occurs by the phase of the LO signal 28, whereinthe result of the mixing is a digitally coded number of invertersignals. The functionality of the controller 34 (decoder), especiallywith regard to the logic implemented into the controller 34 will bediscussed referring to FIG. 3B.

Dependent on the combination of the baseband IQ signals 26 with the LOIQ signals 28 a number of switchable capacitor paths 32 a and 32 b areenabled in parallel via the control signals provided to control outputs36 a and 36 b, wherein the number of enabled (and disabled) switchablecapacitor paths 32 a and 32 b is varied over time in order to output atime signal. Consequently, the first capacitor arrangement 30 of the DAC18′ outputs the modulated RF signal (comprising an in-phase component aswell as the quadrature component (cf. baseband signal 26) modulated on acarrier (cf. LO signal 28)) dependent on the number of enabledswitchable elements 40 a and 40 b and dependent on the chronologicalsequence of the combinations of enabled switchable elements 40 a and 40b of the capacitor arrangement 30. This means that DAC 18′ acts as aso-called direct modulator for IQ signals having a capacitor RF-DACcircuit architecture is formed. In other words, the invention implementsthe digital IQ modulator as DAC 18′ based on elementary mixer cells,comprising switchable capacitor paths 32 a and 32 b, of a capacitiveRF-DAC, wherein the mixing function is implemented into the controller34 (decoder). The direct modulator of DAC 18′ may profitably have alarge tuning range (e.g. a tuning range above 500 MHz exceeding theLTE20 requirements), in combination with a frequency modulation in orderof 60 MHz or above (i.e. 4×60 MHz=240 MHz at DCO frequency). Thisenables a large modulation bandwidth used for a carrier aggregation LTEor WiFi 11n. To sum up, the advantage is that the capacitive modulatorof DAC 18′ combines a high bandwidth resulting in a large tuning signalrange with a high efficiency. Since the architecture of the modulator ofDAC 18′ is based on a non-modulated LO path, a further advantage is thatit can also be used in multiple, simultaneous transmitter applicationslike MIMO using a synthesizer.

It should be noted that the shown capacitor arrangement 30 may,according to further implementations, comprise more than the illustratedtwo switchable capacitor paths 30 a and 30 b (e.g. 512 capacitor pathsenabling a 9-bit DA converting).

As described below, the switchable elements may be realized by inverters40 a and 40 b. An inverter 40 a and 40 b typically comprises twotransistors of two different transistor types (e.g. n-type and p-type)coupled to each other such that the control contacts (gate contacts) ofboth are coupled to a common input and an output contacts (draincontacts) of both are coupled to a common output, wherein the supplyvoltage for the inverter circuit is applied between the two respectiveinput contact (source contacts) of transistors. Often, an inverter, alsoreferred to as not-gate, is realized by CMOS technologies, so that thefirst transistor is formed by a P-MOSFET, wherein the second transistoris formed by an N-MOSFET.

FIG. 3A shows a RF DAC 18″ (direct modulator) implemented along with atransmitter 12′ configured for a single-ended transmission. Thetransmitter 12′ comprises an RF-balun 48 and the RF DAC 18″ having acontroller 34′.

The RF-balun 48 comprises a first inductivity 48 a and a first capacity48 b coupled in parallel. The first inductivity 48 a may have nwindings. With this first inductivity 48 a, a second inductivity 58 amay be coupled inductively, wherein the second inductivity 50 a istypically combined with a further second capacity 50 b connected inparallel to same. The second inductivity 50 a may comprise m windings sothat a transformer is formed by the two inductively coupledinductivities 48 a and 50 a. The RF-balun 48 is connected at a firstside to the RF DAC 18″ via a common node 42, wherein the second side maybe left floating. In this implementation, the plurality of switchablecapacitor paths 32 a, 32 b, 32 c and 32 d are coupled to the common node42 via the capacitors 38 a, 38 b, 38 c and 38 d, respectively.

The plurality of elementary mixer cells, comprising switchable capacitorpaths 32 a, 32 b, 32 c and 32 d, are enabled and disabled by thecontroller 34′ via the inverters 40 a, 40 b, 40 c and 40 d. In contrastto the DAC 18′ of FIG. 2 controlled by the controller 34 in order tooutput a 50% duty cycle, the DAC 18″ of FIG. 3A may be controlled by 34′such that signal corresponding to a signal having a 25% duty cycle (ordifferent duty cycles) may be output via the RF-balun 48.

Therefore, the controller 34′ (decoder) comprises four inputs for thebaseband signal 26′, namely one for the in-phase component (I), one forthe inverted in-phase component (IX), one for the quadrature component(Q) and one for the inverted quadrature component (QX). The controller34′ further comprises four inputs for the LO signal 28′, each 90°dephased (cf. LOI, LOIX, LOQ, LOQX), corresponding to I, IX, Q, QX.Thus, the controller 34′ is configured to control the switchablecapacitor paths 32 a, 32 b, 32 c and 32 d of the capacitor arrangement30′, in accordance to a two-part in-phase modulation signal representingthe combination of I, IX, LOI and LOIX and/or in accordance to atwo-part quadrature modulation signal representing the combination of Q;QX, LOQ and LOQX, via binary control signals 36′a, 36′b, 36′c and 36′dsuch that a modulated RF signal corresponding to a 25% duty cycle signalis output to the RF-balun 48. For the sake of completeness, it should benoted that the RF signal modulated onto a carrier signal comprising LOsignal 28′ consequently comprises the in-phase component (I), thequadrature component (Q) and the inverted versions of same (IX, QX).Here, the baseband signal 26′ and the LO signal 28′ may be combined suchthat the common mode is rejected, in more detail, such that a commonmode within the modulated RF signal is reduced or even avoided.

Below, the logic of the controller 34′ for outputting a 25% duty cycleequivalent will be discussed. FIG. 3B shows a possible implementation ofthe logic of the controller 34′ comprising four logicalelements/operations (e.g. XNOR-operations or AND-operations), namely afirst logical element 56 a, a second logical element 56 b, a thirdlogical element 56 c, and a fourth logical element 56 d. The firstlogical element 56 a combines a first baseband signal 26′a (bb_iq) (e.g.a single-bit signal) and a first LO signal 28′a (lo_iq). The secondlogical element 56 b combines a second baseband signal 26′b (bb_ixqx)and a second LO signal 28′b (lo_ixqx). The third logical element 56 ccombines a third baseband signal 26′c (bb_ixq) and a third LO signal28′c (lo_ixq). The fourth logical element 56 d combines a fourthbaseband signal 26′d (bb_iqx) and a fourth LO signal 28′d (lo_iqx). Thefour logical elements 56 a, 56 b, 56 c and 56 d output the respectivecombined signals to a fifth logical element 58 (e.g. to a furtherAND-element). The fifth logical element 58 is configured to combine thereceived signals which represent a combination of the respectivein-phase component and the quadrature-phase component with the in-phaseoscillation signal and the quadrature oscillation signal to obtain thein-phase modulation signal and/or the quadrature modulation signaland/or a signal combining these two signals. In accordance to thesemodulation signals the controller 34′ selectively enables the singularcapacitor paths (not shown) of the capacitor arrangement via binarycontrol signals.

According to further implementations, the shown controller 34′ (decoder)configured to combine the LO signals 28′ and the baseband signals 26′may be formed as a software implemented unit. Thus, the above-describedcombining of the baseband signals 26′ and the LO signals 28′ may be, atleast partially, performed by a processing unit like a CPU.

FIG. 4A shows a further implementation of a DAC 18′″ with a transmitter12″ that is configured to output a differential signal. The shown DAC18′″ (modulator) comprises the first capacitor arrangement 30′comprising the plurality of switchable capacitor paths 32 a, 32 b, 32 cand 32 d which are coupled via the common node 42. The switchablecapacitor paths 32 a, 32 b, 32 c and 32 d of the first capacitorarrangement 30′ are controlled via the controller 34′, as explainedabove.

The DAC 18′″ further comprises a second capacitor arrangement 60, whichis substantially equal to the first capacitor arrangement 30′, whereinthe switchable capacitor paths 32 a, 32 b, 32 c and 32 d are coupled toa second common node 62. The two capacitor arrangements 30′ and 60′ arecoupled to each other via the RF-balun 48. For example, the firstcapacitor arrangement 30′ is connected to the RF-balun 48 at a firstside via the first common node 42, wherein the second capacitorarrangement 60 is connected to the RF-balun 48 to the second side viathe second common node 62. The second capacitor arrangement 60 iscontrolled via a second controller 64 which is also substantiallysimilar to the first controller 34′.

The controlling of the second capacitor arrangement 60 is based on thebaseband signal 26′ and the LO signal 28′. The second controller 64 isconfigured to receive the baseband signal 26′ comprising the in-phasecomponent and the quadrature component as well as the LO signal 28′,wherein the terminal assignment may differ from the first capacitorarrangement 30′. The second capacitor arrangement 60 should becontrolled such that the RF signal output via the second common node 62is (regarding its value) similar or even equal to the RF signal outputby the capacitor arrangements 30′ via the first common node 42, butinverted. Thus, the signal applied between the two common nodes 42 and62 is a differential signal. The logic of the controller 64 forcontrolling the second capacitor arrangement 60 will be discussed indetail referring to FIG. 4B.

FIG. 4B illustrates logical combinations of the two controllers 34′ and64. The logical combinations of the first controller 34′ correspond tothe logical combinations discussed referring to FIG. 3B. The secondcontroller 64 also comprises four logical elements 66 a, 66 b, 66 c and66 d (e.g. AND-operators) and a fifth logical element 68 which isconnected to the four logical elements 66 a, 66 n, 66 c and 66 d. Thefirst logical element 66 a is configured to combine the second basebandsignal 26′b (bb_ixqx) and the first LO signal 28′a (lo_iq). The secondlogical element 66 b is configured to combine the first baseband signal26′a (bb_iq) and the second LO signal 28′b (lo_ixqx). The third logicalelement 66 c is configured to combine the fourth baseband signal 26′d(bb_iqx) and the third LO signal 28′c (lo_ixq). The fourth logicalelement 66 d is configured to combine the fourth baseband signal 26′c(bb_ixq) and the fourth LO signal 28′d (lo_iqx). These four signals arecombined by the fifth logical elements 68 to output a combination of thein-phase modulation signal and/or of the quadrature-phase modulationsignal of based on which the controlling of the second capacitorarrangement (not shown) may be performed. Thus, the four logical element66 a, 66 b, 66 c and 66 d combine the 180° de-phased versions of thebaseband signals 26′ (cf. logical elements 56 a, 56 b, 56 c and 56 d).

According to further implementation, the control logic may be configuredto combine a plurality of baseband signals with a plurality of LOsignals in parallel, such that the RF signal output by the plurality ofelementary mixer cells, comprising switchable capacitor paths 32 a, 32b, 32 c and 32 d, comprises a plurality of component carriers (e.g. incase carrier aggregation). Starting from the structure of controller34′, shown by FIG. 3A, the controller 34′ (decoder) receives a firstbaseband signal 26′a comprising an in-phase component I and a quadraturecomponent Q and a second baseband signal 26′b comprising a furtherin-phase component IX and a further quadrature component QX, andcombines these two baseband signals 26′a and 26′b with two LO signals28′a and 28′b. In detail, the first baseband signal 26′a is combinedwith the first LO signal 28′a comprising an in-phase LO signal LOI andan quadrature LO signal LOQ, wherein the second baseband signal 26′b iscombined with the second LO signal 28′b comprising an in-phase LO signalLOIX and an quadrature LO signal LOQX. Thus, the modulation of twobaseband signals 26′a and 26′b by using two LO signals 28′a and 28′b isperformed in the digital domain.

With respect to the controlling means comprising the controller 34′ andcontroller 64, i.e. that the controlling of the two different capacitorarrangements 30′ and 60 may be based on the same input signals, namelythe baseband signal 26′ and the LO signal 28′, wherein a differentlogical combination of the singular path signals of the baseband signal26′ and the LO signal 28′ is used for the second controller 64 whencompared to the first controller 34′. According to anotherimplementation the two controllers, 34′ and 64, may be combined to acentral controller which controls the two capacitor arrangements.

It should be noted the controlling of the DAC 18″ is not limited to theabove discussed logic, so, according to another implementation of thecontrollers 34′ and 64, also another combination of the singular IQ andLO signals may be feasible, wherein the second controller 64 typicallycombines a de-phased version of the signals combined by the firstcontroller 34′.

According to another implementation, controllers 34, 34′, 64 or thecombined controller comprising the controllers 34′ and 64 may beimplemented as a two-stage version. The two-stage version comprises acentral controller and a plurality of controllers which are arrangednext to a respective switchable capacitor path 32 a, 32 b, 32 c or 32 d.In general, it should be noted that the arrangement of the controllers34, 34′ and 64 is selected such that the signal path between arespective controller 34, 34′ and 64 to a respective inverter 40 a, 40b, 40 c or 40 d to be controlled is minimized.

Furthermore, it should be noted that the controllers 34, 34′ and 64 arenot necessarily arranged within the transmitters 12, 12′ or 12″. Thus,the controllers 34, 34′ and 64 may be, for example, integrated into anupstream unit (e.g. into the baseband processor).

Referring to FIG. 1, it should be noted that the shown transmitter 12may have MIMO capability when of the discussed implementations of DACs18′, 18″ or 18′″ is implemented. Therefore, the controllers 34, 34′, 34″and 64 may be configured to perform the controlling of the capacitorarrangements 30′, 60 such that multiple IQ data may be modulated ontothe carrier output via the respective capacitor arrangements 30′ and 60.

Regarding FIGS. 3A and 4A it should be noted that the capacitances ofthe plurality of capacitors 38 a, 38 b, 38 c and 38 d of the capacitorarrangement 30′ and/or 60 are preferably equal (thermometer codedarrangement). According to a further implementation, the capacitorarrangements 30′ and/or 60 may have an additional switchable capacitorpath having a reduced capacitance in order to enable a DA convertinghaving an increased accuracy. The capacitance of this additionalswitchable capacitor path may, for example, be equal to the half of thecapacitance of the other capacitors 38 a, 38 b, 38 c and 38 d. Accordingto another implementation, the capacitor arrangement 30′ and/or 60 maycomprise further switchable capacitor paths having a further reducedcapacitance (e.g. a quarter of the original capacitance of one of thecapacitors 38 a, 38 b, 38 c and 38 d) for further increasing theaccuracy of the DA converting. According to another implementation acapacitance of each of the plurality of capacitors 38 a, 38 b, 38 c and38 d of the capacitor arrangements 30′ and/or 60 may increase from onecapacitor to a subsequent capacitor, e.g. by a factor of 2 (binary codedarrangement).

Although, in the above implementations the capacitor arrangement 30′and/or 60′ is illustrated as a capacitor arrangement comprising 512capacitor paths. It should be noted that also more or less capacitorpaths may be provided, wherein the resolution of the digital to analogconverting depends on the number of capacitor paths.

Also, in the above implementations the digital to analog converters 18,18′, 18″ and 18′″ are shown as a modulator which is configured tomodulate a RF signal based on an IQ signal, it should be noted that theshown modulator may also have a second mode in which the RF signaloutput by the DACs 18, 18′, 18″ or 18′″ is based on polar signals,wherein the phase signal is provided as a phase shift in the LO signal.Thus, the controllers 34, 34′ and 62′ may be configured to control theswitchable capacitor paths 32 a, 32 b, 32 c or 32 d based on a vectorsignal in the first mode and based on a polar signal in the second mode.

Although some aspects have been described in the context of anapparatus, it is clear that these aspects also represent a descriptionof the corresponding method for operating a DAC or for controlling acapacitor arrangement 30, 30′ and/or 60 of DAC or, in general, forconverting an analog signal to a digital signal. Here, a block or devicecorresponds to a method step or a feature of a method step. Analogously,aspects described in the context of a method step also represent adescription of a corresponding block or item or feature of acorresponding apparatus. Some or all of the method steps may be executedby (or using) a hardware apparatus, like for example, a microprocessor,a programmable computer or an electronic circuit. In someimplementations, some one or more of the most important method steps maybe executed by such an apparatus.

Depending on certain implementation requirements, implementations of theinvention can be implemented in hardware or in software. Theimplementation can be performed using a digital storage medium, forexample a floppy disk, a DVD, a Blu-Ray, a CD, a ROM, a PROM, an EPROM,an EEPROM or a FLASH memory, having electronically readable controlsignals stored thereon, which cooperate (or are capable of cooperating)with a programmable computer system such that the respective method isperformed. Therefore, the digital storage medium may be computerreadable.

Some implementations according to the invention comprise a data carrierhaving electronically readable control signals, which are capable ofcooperating with a programmable computer system, such that one of themethods described herein is performed.

Generally, implementations of the present invention can be implementedas a computer program product with a program code, the program codebeing operative for performing one of the methods when the computerprogram product runs on a computer. The program code may for example bestored on a machine readable carrier.

Other implementations comprise the computer program for performing oneof the methods described herein, stored on a machine readable carrier.

In other words, an implementation of the inventive method is, therefore,a computer program having a program code for performing one of themethods described herein, when the computer program runs on a computer.

A further implementation of the inventive methods is, therefore, a datacarrier (or a digital storage medium, or a computer-readable medium)comprising, recorded thereon, the computer program for performing one ofthe methods described herein. The data carrier, the digital storagemedium or the recorded medium are typically tangible and/ornon-transitionary.

A further implementation of the inventive method is, therefore, a datastream or a sequence of signals representing the computer program forperforming one of the methods described herein. The data stream or thesequence of signals may for example be configured to be transferred viaa data communication connection, for example via the Internet.

A further implementation comprises a processing means, for example acomputer, or a programmable logic device, configured to or adapted toperform one of the methods described herein.

A further implementation comprises a computer having installed thereonthe computer program for performing one of the methods described herein.

A further implementation according to the invention comprises anapparatus or a system configured to transfer (for example,electronically or optically) a computer program for performing one ofthe methods described herein to a receiver. The receiver may, forexample, be a computer, a mobile device, a memory device or the like.The apparatus or system may, for example, comprise a file server fortransferring the computer program to the receiver.

In some implementations, a programmable logic device (for example afield programmable gate array) may be used to perform some or all of thefunctionalities of the methods described herein. In someimplementations, a field programmable gate array may cooperate with amicroprocessor in order to perform one of the methods described herein.Generally, the methods are preferably performed by any hardwareapparatus.

The above described implementations are merely illustrative for theprinciples of the present invention. It is understood that modificationsand variations of the arrangements and the details described herein willbe apparent to others skilled in the art. It is the intent, therefore,to be limited only by the scope of the impending patent claims and notby the specific details presented by way of description and explanationof the implementations herein.

The invention claimed is:
 1. A radio frequency digital to analogconverter, comprising: a first capacitor arrangement comprising aplurality of switchable capacitor paths arranged in parallel, whereineach switchable capacitor path comprises a switchable element and acapacitor coupled in series; a first common node connected to theplurality of switchable capacitor paths of the first capacitorarrangement; and a first controller configured to receive a basebandsignal having an in-phase component and a quadrature component, and toreceive an LO (local oscillator) signal having an in-phase LO signal anda quadrature LO signal, wherein the first controller is configured tocombine the in-phase component and the in-phase LO signal to obtain afirst in-phase modulation signal, and to combine the quadraturecomponent and the quadrature LO signal to obtain a first quadraturemodulation signal, wherein the first controller is configured to controlthe plurality of switchable capacitor paths of the first capacitorarrangement in accordance with the first in-phase modulation signal orin accordance with the first quadrature modulation signal.
 2. The radiofrequency digital to analog converter according to claim 1, wherein theswitchable element comprises an inverter.
 3. The radio frequency digitalto analog converter according to claim 1, wherein the radio frequencydigital to analog converter is configured to output a modulated in-phasesignal or a modulated quadrature signal via the first common node,wherein a value of the modulated in-phase signal or a value of themodulated quadrature signal depends on a number of enabled switchableelements of the first capacitor arrangement.
 4. The radio frequencydigital to analog converter according to claim 3, wherein the firstcontroller controls the switchable elements such that the modulatedin-phase signal and the modulated quadrature signal are output inaccordance with an averaged duty cycle of 50%.
 5. The radio frequencydigital to analog converter according to claim 1, further comprising anRF-balun connected to the first common node via a first side of theRF-balun.
 6. The radio frequency digital to analog converter accordingto claim 5, wherein the RF-balun comprises at least one inductivity andat least one capacity connected in parallel.
 7. The radio frequencydigital to analog converter according to claim 1, wherein the in-phasecomponent comprises two in-phase component parts, wherein the quadraturecomponent comprises two quadrature component parts, wherein the in-phaseLO signal comprises two in-phase LO signal parts, and wherein thequadrature LO signal comprises two quadrature LO signal parts.
 8. Theradio frequency digital to analog converter according to claim 7,wherein the first controller is configured to combine the two in-phasecomponent parts and the two in-phase LO signal parts, and to combine thetwo quadrature component parts and the two quadrature LO signal partssuch that modulated in-phase signals or modulated quadrature signalsoutput via the first common node are output.
 9. The radio frequencydigital to analog converter according to claim 1, wherein the firstcontroller is configured to combine a further in-phase component to bemodulated onto a further carrier and a further in-phase LO signal toobtain an further in-phase modulation signal, and to combine a furtherquadrature component to be modulated onto the further carrier and afurther quadrature LO signal to obtain a further quadrature modulationsignal.
 10. The radio frequency digital to analog converter according toclaim 1, wherein the plurality of switchable capacitor paths is coupledto the first common node via the respective capacitor.
 11. The radiofrequency digital to analog converter according to claim 1, wherein eachcapacitor of the plurality of the capacitor paths has an equal capacity.12. The radio frequency digital to analog converter according to claim1, wherein a capacitor of a second of the plurality of capacitor pathshas a capacity which is increased over a capacity of a previous one ofthe plurality of capacitor paths.
 13. The radio frequency digital toanalog converter according to claim 1, wherein at least one capacitor ofone of the plurality of switchable capacitor paths has a reducedcapacity when compared to a capacity of another capacitor of another ofthe switchable capacitor paths, wherein the reduced capacity is reducedby at least 25%.
 14. The radio frequency digital to analog converteraccording to claim 1, wherein the first controller comprises one centralcontroller and a plurality of sub-controllers, which are arrangedbetween the central controller and the respective switchable element tobe controlled.
 15. The radio frequency digital to analog converteraccording to claim 1, which is configured to output a single endedanalog signal via the first common node in accordance with the basebandsignal.
 16. The radio frequency digital to analog converter according toclaim 15, wherein the single ended analog signal is modulated to acarrier frequency which depends on the LO signal.
 17. The radiofrequency digital to analog converter according to claim 1, comprising:a second capacitor arrangement comprising a plurality of switchablecapacitor paths arranged in parallel, wherein each switchable capacitorpath comprises a switchable element and a capacitor coupled in series; asecond common node connected to the plurality of switchable capacitorpaths of the second capacitor arrangement; and a second controllerconfigured to receive the baseband signal and the LO signal, wherein thesecond controller is configured to combine the in-phase component andthe in-phase LO signal to obtain a second in-phase modulation signal,and to combine the quadrature component and the quadrature LO signal toobtain a second quadrature modulation signal, and wherein the secondcontroller is configured to control the plurality of switchablecapacitor paths of the second capacitor arrangement in accordance withthe second in-phase modulation signal or in accordance with the secondquadrature modulation signal.
 18. The radio frequency digital to analogconverter according to claim 17, wherein the radio frequency digital toanalog converter is configured to output a differential analog signalbetween the first and the second common node in accordance with thebaseband signal.
 19. The radio frequency digital to analog converteraccording to claim 17, further comprising an RF-balun arranged betweenthe first and the second common node.
 20. The radio frequency digital toanalog converter according to claim 17, wherein the first controller isdifferent from the second controller or wherein a terminal assignment ofinput terminals for receiving the LO signal of the first controllerdiffers from a terminal assignment of input terminals for receiving theLO signal of the second controller.
 21. The radio frequency digital toanalog converter according to claim 17, wherein the first controller andthe second controller are integrated into a central controller.
 22. Aradio frequency digital to analog converter comprising: a firstcapacitor arrangement comprising a plurality of switchable capacitorpaths and a second capacitor arrangement comprising a further pluralityof switchable capacitor paths, each switchable capacitor path comprisesan inverter and a capacitor coupled in series, wherein the switchablecapacitor paths of the first capacitor arrangement are arranged inparallel and coupled to a first common node via respective capacitorsand wherein the switchable capacitor paths of the second capacitorarrangement are arranged in parallel and coupled to a second common nodevia the respective capacitors; an RF-balun arranged between the firstcommon node and the second common node; and a first controller and asecond controller, wherein the first controller and the secondcontroller are configured to receive a baseband signal having anin-phase component and a quadrature component, and to receive an LOsignal having an in-phase LO signal and a quadrature LO signal, whereinthe first controller is configured to combine the in-phase component andthe in-phase LO signal to obtain a first in-phase modulation signal, andto combine the quadrature component and the quadrature LO signal toobtain a first quadrature modulation signal, and wherein the firstcontroller is configured to control the plurality of inverters of thefirst capacitor arrangement in accordance with the first in-phasemodulation signal or in accordance with the first quadrature modulationsignal, wherein the second controller is configured to combine thein-phase component and the in-phase LO signal to obtain a secondin-phase modulation signal, and to combine the quadrature component andthe quadrature LO signal to obtain a second quadrature modulationsignal, and wherein the second controller is configured to control theplurality of inverters of the second capacitor arrangement in accordancewith the second in-phase modulation signal or in accordance with thesecond quadrature modulation signal; wherein the radio frequency digitalto analog converter is configured to output a differential analog signalcomprising the in-phase component and the quadrature component betweenthe first and the second common node in accordance with the basebandsignal.
 23. A transmitter of a mobile communication device comprising:an antenna; and a radio frequency digital to analog converter coupled tothe antenna, wherein the radio frequency digital to analog convertercomprises: a first capacitor arrangement comprising a plurality ofswitchable capacitor paths and a second capacitor arrangement comprisinga further plurality of switchable capacitor paths, each switchablecapacitor path comprises an inverter and a capacitor coupled in series,wherein the switchable capacitor paths of the first capacitorarrangement are arranged in parallel and coupled to a first common nodevia respective capacitors and wherein the switchable capacitor paths ofthe second capacitor arrangement are arranged in parallel and coupled toa second common node via the respective capacitors; an RF-balun arrangedbetween the first common node and the second common node; and a firstcontroller and a second controller, wherein the first controller and thesecond controller are configured to receive a baseband signal having anin-phase component and a quadrature component, and to receive an LOsignal having an in-phase LO signal and a quadrature LO signal, whereinthe first controller is configured to combine the in-phase component andthe in-phase LO signal to obtain a first in-phase modulation signal, ando combine the quadrature component and the quadrature LO signal toobtain a first quadrature modulation signal, and wherein the firstcontroller is configured to control the plurality of inverters of thefirst capacitor arrangement in accordance with the first in-phasemodulation signal or in accordance with the first quadrature modulationsignal, wherein the second controller is configured to combine thein-phase component and the in-phase LO signal to obtain a secondin-phase modulation signal, and to combine the quadrature component andthe quadrature LO signal to obtain a second quadrature modulationsignal, and wherein the second controller is configured to control theplurality of inverters of the second capacitor arrangement in accordancewith the second in-phase modulation signal or in accordance with thesecond quadrature modulation signal; wherein the radio frequency digitalto analog converter is configured to output a differential analog signalcomprising the in-phase component and the quadrature component betweenthe first and the second common node in accordance with the basebandsignal.
 24. A method for converting digital signal to an analog signal,comprising: receiving a baseband signal having an in-phase component anda quadrature component, and receiving an LO signal having an in-phase LOsignal and a quadrature LO signal; combining the in-phase component andthe in-phase LO signal to obtain a first in-phase modulation signal, andcombining the quadrature component and the quadrature LO signal toobtain a first quadrature modulation signal; and controlling a pluralityof switchable capacitor paths of a first capacitor arrangement inaccordance with the first in-phase modulation signal or in accordancewith the first quadrature modulation signal, wherein the first capacitorarrangement comprises the plurality of switchable capacitor pathsarranged in parallel, wherein each switchable capacitor path comprises aswitchable element and a capacitor coupled in series, and wherein theplurality of switchable capacitor paths of the first capacitorarrangement is connected via a first common node.
 25. The methodaccording to claim 24, further comprising: outputting the analog signalvia the first common node in accordance with the baseband signal. 26.Method for converting digital signal to an analog signal, comprising:receiving a baseband signal having an in-phase component and aquadrature component, and receiving an LO signal having an in-phase LOsignal and a quadrature LO signal, wherein the in-phase componentcomprises two in-phase component parts, wherein the quadrature componentcomprises two quadrature component parts, wherein the in-phase LO signalcomprises two in-phase LO signal parts, and wherein the quadrature LOsignal comprises two quadrature LO signal parts; combining a first partof the in-phase component and a first part of the in-phase LO signal toobtain a first in-phase modulation signal, and combining a second partof the in-phase component and a second part of the in-phase LO signal toobtain a second in-phase modulation signal; combining a first part ofthe quadrature component and a first part of the quadrature LO signal toobtain a first quadrature modulation signal, and combining a second partof the quadrature component and a second part of the quadrature LOsignal to obtain a second quadrature modulation signal; controlling aplurality of switchable capacitor paths of a first capacitor arrangementin accordance with the first in-phase modulation signal or in accordancewith the first quadrature modulation signal, wherein the first capacitorarrangement comprises the plurality of switchable capacitor pathsarranged in parallel, wherein each switchable capacitor path comprises aswitchable element and a capacitor coupled in series, and wherein theplurality of switchable capacitor paths of the first capacitorarrangement is connected via a first common node; controlling aplurality of switchable capacitor paths of a second capacitorarrangement in accordance with the second in-phase modulation signal orin accordance with the second quadrature modulation signal, wherein thesecond capacitor arrangement comprises the plurality of switchablecapacitor paths arranged in parallel, wherein each switchable capacitorpath comprises a switchable element and a capacitor coupled in series,and wherein the plurality of switchable capacitor paths of the secondcapacitor arrangement is connected via a second common node; andoutputting the analog signal as differential signal between the firstand second common node in accordance with the baseband signal.